The present application claims priority to Japanese Application No. P11-261177 filed Sep. 14, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a focal point position control method and a focal point position control mechanism for auto-focussing an objective lens. This invention also relates to a method and apparatus for inspecting the appearance of a semiconductor wafer.
2. Description of Related Art
A semiconductor device is produced by forming a fine device pattern on a semiconductor wafer. If, in a manufacturing process for a semiconductor device, there occurs an attachment of contaminants, pattern defects or unusual dimensions, defects are produced in the device pattern. The semiconductor device, suffering from these defects, produces rejects to lower the yield in the production process.
For maintaining the yield in the manufacturing process at a high level, it is necessary to find defects ascribable to the contaminants, pattern defects or unusual size, at an earlier stage, to locate the causes and to take effective measures for the manufacturing process. If the cause of the defects is determined quickly to take proper measures in the manufacturing process to improve the yield, it is possible to start a new process quickly to obtain a high yield in the process.
If a defect is produced in a semiconductor device, the defect is detected, using a microscopic device for inspecting the semiconductor, to search the cause of the defect, and, from the result of that search, specify the equipment or the process responsible for the defect. This microscopic device for inspecting the semiconductor is a device, such as an optical microscope, that is able to enlarge the defect on the semiconductor wafer for inspection or to image the enlarged defect to demonstrate the image on a monitor. By using this microscope device for inspecting the semiconductor, it becomes possible to discriminate the sort of the defect produced on the reject device.
According to the current design rule for the semiconductor manufacture process, the patterns prevalently have a line width of 0.18 xcexcm, which tends to be even finer, such as 0.15 xcexcm or 0.13 xcexcm. In keeping pace with the tendency to use a finer design rule in the semiconductor process, fine defects which could be discounted in the past now may raise problems, requiring smaller defects to be detected.
Therefore, in a microscopic device for semiconductor inspection, an objective lens with a higher multiplying factor is needed in order to allow for observation of these fine defects.
However, an objective lens with a high multiplying factor has an extremely short depth of focus. For example, if the numerical aperture (NA) is 0.9, and the multiplying factor 100, the depth of focus is xc2x10.5 xcexcm or less. It is extremely difficult to adjust the focal point position with a short depth of focus by a manual operation each time the inspection is executed. Thus, with the microscopic device for semiconductor inspection, such a mechanism is required which effects auto-focussing accurately and speedily without using a manual operation.
The conventional microscopic device for semiconductor inspection is provided with a distance detection mechanism for detecting the distance between the semiconductor wafer and the objective lens by causing the laser light or the light from an LED to fall on the objective lens through a light probe for measurement and by detecting the reflected light. In the conventional microscopic device for semiconductor inspection, the distance between the objective lens and the semiconductor wafer is controlled, based on the distance information as detected by this distance detection mechanism, in order to bring the focal point position of the objective lens into coincidence with the surface of the semiconductor wafer to be observed to enable auto-focussing.
An auto-focussing mechanism 100, used in such conventional microscopic device for semiconductor inspection, is shown in FIG. 15.
The auto-focussing mechanism 100 includes a stage 102, for supporting a semiconductor wafer 101 to be inspected, a laser diode 103 for radiating the laser light, an objective lens 104 for condensing the laser light radiated from the laser diode 103 to illuminate the semiconductor wafer 101, and a photodetector 105 for receiving the laser light reflected by the semiconductor wafer 101, as shown in FIG. 15.
The conventional auto-focussing mechanism 100 also includes a halfmirror 106 for separating the optical path for the outgoing light from the laser diode 103 from that of the reflected light from the semiconductor wafer 101, a knife edge 107 provided between the laser diode 103 and the halfmirror 106 and a collimator lens 108 provided between the half mirror 106 and the objective lens 104.
This conventional auto-focussing mechanism 100 also includes a pre-amplifier 111 for generating a position detection signal from a detection current of the photodetector 105, and a servo circuit 112 for driving the stage 102 based on the position detection signal from the pre-amplifier 111.
On the stage 102 is placed a disc-shaped semiconductor wafer 101 to be inspected. This stage 102 causes the semiconductor wafer 101, placed thereon, to be moved in the height-wise direction, that is in a direction towards and away from the objective lens 104. The stage 102 is controlled in its movement according to a driving signal supplied from the servo circuit 112.
The laser diode 103 radiates the laser light of a pre-set wavelength. The laser light radiated from the laser diode 103 has its spot shaped to a semicircular profile and is incident in this state on the half mirror 106. The half mirror 106 reflects the laser light radiated from the laser diode 103. The laser light, reflected by the half mirror 106, is collimated by the collimator lens 108 into a parallel light beam which then falls on the objective lens 104. The objective lens 104 converges the collimated laser light to illunminate the semiconductor wafer 101.
The laser spot formed on the semiconductor wafer 101 has a semicircular shape because of the provision of the knife edge 107.
The laser light, converged by the objective lens 104, is reflected by the semiconductor wafer 101, and is again passed through the objective lens 104 and the collimator lens 108 to fall again on the half mirror 106. The half mirror 106 now transmits the reflected light from the semiconductor wafer 101. The reflected light, transmitted through the half mirror 106, is illuminated on the photodetector 105.
The photodetector 105 is conjugated in its arraying position with respect to the laser diode 103. The photodetector 105 receives the reflected light from the semiconductor wafer 101 and converts the reflected light into electrical signals which are routed to the pre-amplifier 111.
The pre-amplifier 111 finds a difference signal, indicating the position of the center of gravity of a laser spot formed on the photodetector 105, from the electrical signals supplied from the photodetector 105. From the difference signal, the pre-amplifier 111 generates a distance detection signal indicating the distance between the semiconductor wafer 101 and the objective lens 104.
The servo circuit 112 drives the stage 102 in the height-wise direction, so that the supplied distance detection signal is equal to a target value, to execute servo control. By setting the target value so as to be equal to the focal length of the objective lens 104, the height-wise position of the semiconductor wafer 101 is brought into coincidence with the focal point position of the objective lens 104.
The principle of the distance detection by this conventional auto-focussing mechanism 100 is hereinafter explained.
The laser spot formed on the semiconductor wafer 101 is of a semicircular profile due to the presence of the knife edge 107. The laser spot formed on the semiconductor wafer 101 is smallest in spot size if the focal point position is coincident with the reflecting surface, as shown in FIG. 16. The farther the reflecting surface is separated from the focal point position, the larger is the spot size. The light spot shape is symmetrical, with respect to spot position as focussed, on both sides of the focal point position.
In this manner, the laser spot formed on the semiconductor wafer 101 has its center-of-gravity position moved linearly in proportion to the amount of deviation of the focal point position. This deviation in the center-of-gravity position reflects itself on the photodetector 105.
Using the photodetector 105, having its light detection area split into plural sub-areas, the deviation in the laser spot receiving position is detected by detecting e.g., the difference in the light volumes received by the respective sub-areas. From this deviation in the center-of-gravity position, the amount of deviation of the focal point position of the laser spot in the semiconductor wafer 101 is found to detect the distance between the semiconductor wafer 101 and the objective lens 104.
In the conventional microscopic device for seminconductor inspection, the focal length between the semiconductor wafer 101 and the objective lens 104 is set, using the above-described auto-focussing mechanism 100. Then, using the objective lens 104, the device pattern on the semiconductor wafer 101 is enlarged and observed.
There is formed on the semiconductor wafer a metal interconnection of aluminum or copper interconnecting the transcribed circuits. The metallic interconnection is generally a stereo interconnection over plural layers. Between the respective interconnection layers, there are formed inter-layer insulating films of, for example, SiO2, to maintain electrical insulation. In the manufacturing process, an inter-layer insulating film is formed to a thickness of approximately 1 xcexcm, and the metallic interconnection is formed thereon. Another inter-layer insulating film and the next metallic interconnection are formed thereon. The above-described process is repeated a number of times. The microscopic device for semiconductor inspection may be used for inspecting line breakages or shorting of the metallic interconnections.
However, the inter-layer insulating film for the semiconductor wafer is usually formed of silicon dioxide, and transmits light. Therefore, if a laser spot is formed on the semiconductor wafer, using the auto-focussing mechanism 100, the distance to the metallic interconnection can be detected if the spot is on the semiconductor wafer, as shown at X in FIGS. 17A and 17B. However, if the spot is on the inter-layer insulating film, as shown at Y in FIGS. 17A and 17B, the reflecting surface is at a lower level than the metallic interconnections, so that it is impossible to detect the distance to the metallic interconnections under inspection.
Thus, in the conventional microscopic device for semiconductor inspection, provided with the auto-focussing mechanism, it has been difficult to automatically focus the objective lens on the metallic interconnections formed on the inter-layer insulating film.
It is therefore an object of the present invention to provide a focal point position control mechanism and a focal point position control method for automatic focussing of the objective lens, and a semiconductor wafer inspecting method and apparatus for automatic focussing of the objective lens and for inspecting the appearance of the semiconductor wafer.
In one aspect, the present invention provides a focal point position control mechanism including means for supporting an object for illumination formed by a light-transmitting material and a light reflecting pattern formed thereon, means for illuminating light converged by an objective lens on the object of illumination supported by the supporting means, means for detecting the distance between the light reflecting pattern and the objective lens, by forming a light spot on the object of illumination using the objective lens and by detecting the reflected light from the light spot thus formed and means for controlling the shifting of the supporting means and/or the objective lens according to the distance between the light reflecting pattern and the objective lens as detected by the distance detection means, for bringing the distance between the light reflecting pattern and the objective lens into coincidence with the focal length of the objective lens. The distance detection means causes the light spot formed on the object of illumination to be oscillated in the horizontal direction with respect to the object of illumination to detect the distance between the light reflecting pattern and the objective lens.
In this focal point position control mechanism, the light spot formed on the object of illumination is oscillated in the horizontal direction relative to the object of illumination, and the distance between the light reflection pattern and the objective lens is detected to bring the focal point position of the objective lens into coincidence with the light reflection pattern forming position.
In another aspect, the present invention provides a method for controlling the focal point position in which a focal point position of an objective lens adapted for illuminating light converged on an object of illumination formed by a light-transmitting material and a light reflecting pattern formed thereon is brought into coincidence with the position in which the reflective pattern is formed. The method includes forming a light spot on the object of illumination using the objective lens, causing oscillations of the light spot formed on the object of illumination in a horizontal direction relative to the object of illumination, detecting the reflected light from the light spot thus formed to detect the distance between the light reflecting pattern and the objective lens and bringing the distance between the light reflecting pattern and the objective lens into coincidence with the focal length of the objective lens according to the distance between the light reflecting pattern and the objective lens as detected.
In this focal point position controlling method, the light spot formed on the object of illumination is oscillated in the horizontal direction relative to the object of illumination, and the distance between the light reflection pattern and the objective lens is detected to bring the focal point position of the objective lens into coincidence with the light reflection pattern forming position.
In still another aspect, the present invention provides an apparatus for inspecting a semiconductor wafer including supporting means for supporting a semiconductor wafer carrying a metallic interconnection, means for illuminating the light converged by an objective lens on a semiconductor wafer carried by the supporting means, means for detecting the distance between the metallic interconnection and the objective lens, by forming a light spot on the object of illumination using the objective lens, and by detecting the reflected light from the light spot, thus formed, control means for controlling the shifting of the supporting means and/or the objective lens according to the distance between the metallic interconnection and the objective lens as detected by the distance detection means for bringing the distance between the metallic interconnection and the objective lens into coincidence with the focal length of the objective lens, photographing means for photographing an image of the semiconductor wafer by detecting the reflected light of the light illuminated by the light illuminating means on the semiconductor wafer, and inspection means for inspecting the semiconductor wafer by processing the image photographed by the photographing means. The distance detection means causes the light spot formed on the semiconductor wafer to be oscillated in the horizontal direction with respect to the semiconductor wafer to detect the distance between the metallic interconnection and the objective lens.
In this semiconductor wafer inspecting apparatus, the light spot formed on the semiconductor wafer is oscillated in the horizontal direction relative to the metallic interconnection, and the distance between the metallic interconnection and the objective lens is detected to bring the focal point position of the objective lens into coincidence with the metallic interconnection forming position. In the present semiconductor wafer inspecting apparatus, the image of the semiconductor wafer is photographed to inspect the semiconductor wafer, as the focal point position of the objective lens is coincident with the light reflection pattern forming position.
In yet another aspect, the present invention provides a method for inspecting a semiconductor wafer in which the light converged by an objective lens is illuminated on a semiconductor wafer carrying a metallic interconnection and the reflected light is detected to inspect the semiconductor wafer. The method includes forming a light spot on the semiconductor wafer using the objective lens, oscillating the light spot formed on the semiconductor wafer relative to the semiconductor wafer, detecting the distance between the metallic interconnection and the objective lens by detecting the reflected light from the light spot formed, bringing the distance between the metallic interconnection and the objective lens into coincidence with the focal length of the objective lens based on the detected distance between the metallic interconnection and the objective lens, converging the light on the semiconductor wafer by the objective lens, photographing an image of the semiconductor wafer by detecting the reflected light of the light converged on the semiconductor wafer, and processing the photographed image to inspect the semiconductor wafer.
In this semiconductor wafer inspecting method, the light spot formed on the semiconductor wafer is oscillated in the horizontal direction relative to the metallic interconnection, and the distance between the metallic interconnection and the objective lens is detected to bring the focal point position of the objective lens into coincidence with the metallic interconnection forming position. In the present semiconductor wafer inspecting apparatus, the image of the semiconductor wafer is photographed to inspect the semiconductor wafer, as the focal point position of the objective lens remains coincident with the light reflection pattern forming position.
In the focal point position controlling method and apparatus, according to the present invention, the light spot formed on the object of illumination is oscillated in the horizontal direction relative to the object of illumination. From the reflected light of the oscillated light spot, the distance between the light reflection pattern formed on the object of illumination and the objective lens is detected to bring the focal point position of the objective lens into coincidence with the light reflection pattern forming position.
In this manner, according to the focal point position controlling method and apparatus of the present invention, the objective lens can be auto-focussed extremely readily. For example, the objective lens can be auto-focussed on a metallic interconnection formed on a light-transmitting inter-layer insulating film.
In the semiconductor wafer inspecting method and apparatus according to the present invention, the light spot formed on the semiconductor wafer is oscillated in the horizontal direction with respect to this semiconductor wafer. From the reflected light of the oscillated light spot, the distance between the metallic interconnection formed on the semiconductor wafer and the objective lens is detected to bring the focal point position of the objective lens into coincidence with the metallic interconnection forming position. In the present semiconductor wafer inspecting method, the image of the semiconductor wafer is photographed to inspect the semiconductor wafer, as the focal point position of the objective lens remains coincident with the light reflection pattern forming position.